Pressure responsive speed controller



May 9, 1950 R. F. WEHRLIN ET AL, 2,507,326

PRESSURE RESPONSIVE SPEED CONTROLLER Filed Oct. 4, 1945 4 Sheets-Sheet l HVV EN TOR'.

May 9, 1950 R. F. WEHRLIN ET AL,

PRESSURE RESPONSIVE SPEED CONTROLLER 4 Sheets-Sheei 2 Filed OGL. 4, 1945 May 9, 1950 R. F. WEHRLIN ET AL 2,597,326

PRESSURE RESPONSIVE SPEED CONTROLLER Filed 0G11. 4, 1945 4 Sheets-Sheet 5 To f "101 m17 .IIIA .li

[Mmm/Q6, 63

May 99 1950 R. F. WEHRLIN ET A1. 2,507,326

PRESSURE RESPONSIVE SPEED CONTROLLER Filed Oct. 4, 1945 4 Sheets-Shed 4 IN VEN TORS.

Patented May 9, 1950 UNITED STATES PATENT OFFICE PRESSURE RESPONSIVE SPEED CONTROLLER necticut Application october 4, 1945, serial No. 620,232

4 claims. (ol. 31a-sos) Our invention relates to pressure responsive speed controllers, and more particularly to pressure responsive speed controllers adapted for use as altimeters, in bombsights, and of general application.

In the prior art, altimeters for airplanes are substantially devices which measure altitude as a function of the absolute pressure existing in the atmosphere surrounding the plane at its altitude. Corrections must be made for barometric pressure and the present instruments become increasingly inaccurate at higher altitudes due to the fact that there is a smaller change in pressure for a given change in altitude at higher altitudes. Furthermore, an absolute motion of the pressure responsive element must take place over a comparatively large range. This results in inaccuracy introduced by lost motion and by hysteresis.

One object of our invention is to provide a pressure responsive device of general application.

Another object of our invention is to provide a pressure responsive device which is accurate over the entire range from sea level to 50,000 feet of altitude or higher if desired.

Another object of our invention is to provide an altimeter which is responsive to extremely small pressure differences.

Another object of our invention is to provide an altimeter in which a scale of any desired amplitude may be employed.

Another object of our invention is to provide an accurate altimeter which may be easily calibrated.

Still a further object of our invention is to provide an altimeter or similar device in which correction is made for the elevation of a landing eld above sea level.

Other and further objects of our invention will appear from the following description.

In the accompanying drawings, which form part of the instant specification, and which are to be read in conjunction therewith, and in which like reference numerals are used to show like parts in various views:

Fig. 1 is a diagrammatic view of apparatus showing one embodiment of our invention.

Fig. 2 is a sectional elevation of a device showing one embodiment of our invention, taken along the line 2--2 of Fig. 3.

Fig. 3 is a sectional elevation of the device shown in Fig. 2 taken along the line 3--3 of Fig. 2.

Fig. 4 is a sectional view taken along the line l-- of Fig. 3.

Fig. 5 is a sectional view taken along the line 5-5 of Fig. 4.

Fig 6 is a, sectional view taken along the line t-G of Fig. 3.

Fig. 7 is a sectional view taken along the line -i oi Fig. 4.

Fig. 8 is a, diagrammatic View showing the electrical connections employed in our invention.

Fig. 9 is a fragmentary perspective view showing a detail of our invention.

In general, our invention contemplates the provision of an electric motor with means for controlling the motors speed inversely proportional to the absolute altitude of the plane in which an altimeter of our invention is positioned. We provide means for controlling the motor to decrease its speed as a function of the pressure of the atmosphere at an altitude which is a predetermined distance above or below an antecedently determined altitude. In other words, as will be more apparent hereinafter, our control is responsive to small pressure differences rather than to the pressure difference between the air pressure at sea level and the air pressure at the altitude of a plane. We provide means for indicating an altitude inversely proportional to the motor speed, that is to say, a high motor speed will indicate a low altitude, while a lower motor speed will indicate a high altitude.

Referring now to Fig. 8, we provide an electric motor i0 having a shaft Il driving a centrifugal governor l2. The arrangement is such that centrifugal force will cause the links of the governor to fly outwardly, moving a rotary sleeve i3 linked to the governor to the right as viewed in Fig. S. A stationary rod I4 is connected to the sleeve I3 by means of a thrust bearing I5. The end of the rod lll is pinned to a lever l1 by means of pin i8. The lever I1 is secured to a shaft IB for movement therewith in any suitable manner, as for example, by machine screws I9. The upper end of lever I'l carries a contact point 20 adapted to make contact with the contact point 2l. The brushes 22 and 23 of the motor lil are connected across any suitable source of potential connected with conductors '24 and 25. The armature circuit of the motor l0 is completed through the contact points 20 and 2|. The field winding 25 of the motor is provided with an adjustable resistance 21. The motor is connected by shaft 23 or in any other suitable manner to the armature of a small generator 29. The eld winding 3.1i of the generator 29 is likewise provided with an adjustable resistance 3l. The output of the generator 29 is connected by conductors 32 and 33 to a galvanometer 3G. The winding of the galvanometer it is such that the pointer 35 thereof is adapted to move clockwise upon an increase in potential across the conductors 32 and 33. In parallel with the galvanometer 34 we place a resistance having a lower branch 36 adapted to varied manually, and an upper branch 3l' adapted to be shunted by means of contact arm 33. lt will be clear to those skilled in the art from the foregoing that the position of Athe galvanometer pointer 35 will be determined by the speed of the generator 29 which in lturn will Vbe determined by the speed of the motor Il). The motor speed in turn is controlled by the moment about the governor shaft I6 against which the centrifugal moment of the governor links acts. The arrangement is such that a high moment around governor shaft l tending to close the circuit of the motor through contact points 29 and 2l will permit the motor to run at a higher speed than when the torque about governor shaft is reduced. As soon as the centrifugal force exercised by the governor overcomes the moment about the governor shaft, the contact point 2t will move away from contact point 2l, opening the circuit and tending to slow down the motor. In actual practice, the motor armature receives a series of impulses approximately 20 per second. The motion of the contact point is extremely small, being between .06 to .003 of an inch. The motor inertia is such that avery smooth and very accurate control is achieved as a function of the moment applied by the governor shaft l5.

` Our control system is directed to controlling the moment applied by the governor shaft i6, thus controlling the motor speed and all functions of motor speed which in the case of an altimeter would be altitude.

Referring now to Fig. l., a shaft it is iournalled in a bearing 4U mounted in a suitable support si. A calibrated drum t2 is secured to the shaft for rotation therewith. 'Mounted above the drum [i2 is a suitable indicator 43 adapted to show the position to which the drum is moved. Likewise secured to the shaft 39 for rotation therewith is a pinion it adapted to mesh with a gear seg ment The gear segment is secured to a shaft te for rotation therewith. Y An arm 41 is secured to shaft it for rotation therewith, as is also an arm iii. The arm '48 carries a pin lis adapted to engage one end 5t of a lever 5I. The control shaft it has secured to it an arm 52 and an arm The arm 53 is provided with a slot in which a pin 55 is lodged. The pin 55 is carried by a link 55, the other end of which is .u in to therarrn 52 forms the other end'of the lever 5i. The lever 5i is pivot'- ally secured to a member 59 by means of screw tti. A spring o! urges the end 58 of the lever 5i upwardly, as can readily be seen by reference to Figs. l and 4. A helical spring 62 is stretched between arm 52 secured to the governor shaft and arm il secured to shaft 4t, as can readily Vbe seen by reference lto Figs. 1 and 5. Referring new to Fig. 6, one end of spring 62 is secured to the arm si by a pin 63. spring S2 is secured to a member 'lo adapted to be nested within the coils of the spring 52. The member iiiV is internally threaded and adapted to receive a screw 65. The screw 65 is secured to a bracket F35 by means of nut 6? and the screw The bracket BE .is pinned by means of pin 6e to the arm 131. Adjustment of the tension of spring t2 may be` made by means of lengthening or shortening the position of the- The other end of' screw $5 with respect to the member and the bracket to. Adjustment may also be made by deadening a portion of the end turns of the spring by means of the member 10.

The tension ofthe spring G2 is transmitted to governor shaft it through the arm 52 tending to rotate the governor shaft in a clockwise direction, opposing the centrifugal force exercised by the governor i2. It will be noted that the construction is such that when the shaft iii is rotated counter-clockwise, the spring tension is lessened and the effective length of the arm il is shortened, thus raising the end of Lie spring Si which is attached to arm fil. This increases the angularity of application of the tension of he spring on arm 52. Contrariwise, a lengthening of the spring tends to increase the effective lever arm of its application. The moment apM plied to the governor shaft thus depends on changes of spring length and changes of effective lever arm length. The construction is such that the angle of the arm, direction of pull, and magnitude of force tend to compensate, thus producing sensitivity. The use of a helical spring with the construction shown enables us to use calibrations of a practically linear function on the altitude drum il?.

Referring again to Fig. l, we provide a pair of Sylphon bellows ii and 'l2 adapted to act on the'opposite ends of lever 'i3 which is secured to a shaft ifi Vmounted in suitable bearings .'5 and it as can readily be seen by reference to Fig. 3. The upper ends of the ybellows are sealed and their interiors are in communication at their lower ends with a manifold 'll which cornmunicates through a conduit 'it with a itting 79 which is connected by suitable piping to the airplane Pitot-static line (not shown). Communication with the interior of bellows 'i2 with the manifold il is controlled by a valve se controlled by a control knob 8i. A small bellows 82 seals the Pitot-static line from pressure ccnditions within the plane. A spring loads the valve .and tends to keep it in seated position. The valve structure is shownI in detail in Fig. 2. The valve t@ is secured to a valve stern tri which is internally threaded. The knob 8| controls a screw Rotation of the screw to move the valve to closed position collapses the bellows 52. Moving the valve to open position is done against the action of the spring $3 by rotation of the control knob Si. The ends of the bellows rocker lever i3 are secured to connecting rods Bt si by pins St and 85%. The connecting rods are threadedly secured to bushings 9s which are in turn threadedly lodged in the closure plates 9! of the bellows. The bushings Se are formed at their upper ends with hexagonal portions 92. to provide for adjustment of the linkage between the bellows and the rocker lever '53. The bellows are supported by a framework 93 which is secured by means of stud kbolts and 95 to a casing Sie in which the mechanism is lodged, as can be seen by reference t0 Fig. 3.

An arm Si' is secured to the bellows shaft llifor rotation therewith. IThis arm is formed with a slotY S8 through which the pin projects. Motion of the bellows shaft 'Erl is transn mitted through arm Si and pin 55 to the arm 53. Referring now to Figs. l and 3, it will be observed that the position of pin 55 governs the proportion of torque on the bellows shaft to the moment produced on the governor shaft by spring 62. When the pin 55 is toward the upper end of slots 5t and 98, we have in effect a short lever arms? acting through a long lever arm 53. This increases the effect of bellows force on the governor shaft. When the pin 55 is moved downwardly, the length or the arm 5l with respect to the length of the effective arm 53 is increased, thus reducing the effect of torque on the bellows shaft ifi. Since the lever arm 52 through which the spring @E acts is substantially iixed, the ratio of bellows effect to spring effect is determined by the position of pin 55. Referring now to e, 5, and 7, it will be recalled that the lever 5! is pivotally mounted around screw upon a member 5S. The member 5S is in turn pivoted around pivot screw 95 secured to the support iii. The upper end of member 55 carries a vertical flexible member mil, adjacent the upper extremity of which is secured a pin iii! adapted to seat in a plurality of holes ill?. Below the pin lili, there is secured to member itt a it. Around the pin 63 :between member liiil and the cover plate iii/l, we position a spring' 55 normally urging the ilexible member' itt into position where the pin lili is retained one ci the holes H152. The pin lille passes through a slot formed in the cover plate los. A handle lill carrying an indicating pointer it is provided so that the pin m3 may be pulled to the right, as viewed in Fig. 5, against the action of spring U35, moving the pin lili out of one or the holes |52. In this maner, the extension lili! may be rocked to the right and to the left, thus oscillating member 59 around its pivot 99. It will be observed that movement oi member ille to the right as viewed in Fig. 4 will rotate member 59 in a clockwise direction. Since the pivot screw 55 is to the left or" the pivot screw 59, it will be raised when the member is moved in a clockwise direction. Since the end 5d of lever' 5i is resting :against the raising of pivot screw 53 will raise the right hand side 55 of the lever 5i. This in turn will raise the linie 5%, moving the pin 55 upwardly in the slot to a position in which the greater effect is given to the torque generated by bellows shait lli.

ln operation, let Us assume that a plane equipped with our invention is to take oil from a landing iield whose elevation is two thousand feet above sea level. The external circuit to our pressure responsive device is closed, thus energiaing the conductors fifi and This starts the motor The drum d2 is set so that Zero altitude api qars under the indicator thus placing a predetermined tension on the spring 52. This tension is communicated by arm 5E to the governor control. shaft i5, the force being such as to tend to close contact point 2li upon contact point 2l. When the motor reaches the predetermined speed, the governor i2 will operate to maintain it at that speed. The movement of the drum is communicated to the shaft 3Q thence to bevel gear lue which meshes with a bevel gear itil. The bevel gear ll is secured to the shaft iii which carries the arm 35. The other end of shaft il! carries a bevel gear i i5 which meshes with a bevel gear H5 which is secured to a shaft ill. The shaft il? is secured to a rotary dial H2 mounted behind a cover plate lit of the altimeter instrument face. The cover plate is provided with an opening H4 through which the Igalvanorneter needle 35 is visible. The amplitude of motion of the galvanorneter needle is but a segment of the cover plate l i3 corresponding to the opening H4. In this manner we may use a comparatively small needle rnum spring tension on the spring with a comparatively large dial face. The large dial face enables us to calibrate altitude in small increments. When the motor reaches the predetermined speed, let us say 50G revolutions per minute by Way of example, a predetermined voltage will be generated by the generator 25. At zero altitude, most of the resistance 3l is shunted by the arm 38 so that only the residual resistance 36 is eiective. The arrangement is such that the current will flow through both the shunt resistance 36 and the winding of the galvanometer 34. At the speed determined by the spring 62, the voltage produced is such that the galvanometer needle 35 will swing to the right as viewed in Figs. 8 and 9, registering a zero altitude. At zero altitude the maximum spring tension is placed on spring 52 as the drum 42 is moved in a counter-clockwise direction. Movement oi the drum 42 in a counter-clockwise direction moves the shaft IH in a clockwise direction, thus cutting out a portion of the shunt resistance 3l. The rotation of the shaft I i l in a clockwise direction moves shaft lil in a counter-clockwise direction, thus moving the zero altitude calibration of the dial H2 toward the right oi the segmental opening iii, as can be seen by reference to Fig. 9.

The rotation of the drum 112 in a counterclockwise direction to bring the zero altitude calibration in register with the indicator 43 rotates the shaft 35 in a counter-clockwise direction. This rotates gear segment 45 and shaft e5 in a clockwise direction, moving the arm el to the left as viewed in Figs. l and 6, thus placing maxi- The arm 4B is likewise moved in a clockwise direction as viewed in Figs. l and 4. This drops the pin :i9 to its lowerrnost position, permitting the spring 6I to raise the right hand side 53 of lever 5l due to the fact that the left hand side 59 oi lever 5i rests on the pin The lowering of the pin 49 will therefore raise pin giving a greater effect to the bellows force. Conversely, when the drum 42 is turned to a higher altitude, moving in a clockwise direction, the arm il will be moved in a counter-clockwise direction, reducing the tension on the spring 62 and increasing its angularlty, thus reducing the effective lever arm of its application to control shaft i5. At the same time, the arm [3S will move in a counter-clockwise direction, raising the pin 45. The raising of the pin lli) will result in a lowering of the pin 55, thus reducing the movement applied to the governor' shaft through arm 53 on account of torque around the bellows shaft l5. This correction is necessary to get true readings of altitude. We have seen that the moment opposing the centrifugal governor depends upon changes of spring length and changes of effective lever arm length. After the altitude drum e2 is set to zero altitude, the valve iii! is closed. As the plane moves to a higher altitude, the pressure within bellows li becomes less, while the pressure at the ground is sealed within bellows 12. This produces a clockwise force on the bellows shaft 74, thus applying a force through the pin 55 to the arm 53 against the force of the spring 62 tending to rotate the control shaft I6 in a counter-clockwise direction tending to open contact point Zil, thus reducing the motor speed. The lower motor speed will produce a lower voltage, causing the galvanoineter needle y35 to move in a counter-clockwise direction, registering a higher altitude on the dial ii?. It will be-seen that thepressure diiierence between the pressure `existing in bellows 12 and sealedY therein by the valve and the pressure of the surrounding atmosphere .as communicated to bellows -l through the Pitot-static line will determine theV motor speed and hence the altitudeY reading. The relationship between a difference in 'motor .speed with respect to a diierenoe of pressure, in other words Ythe ratio dii/dp is the value with which we are concerned. Actually, this ratio changes slowly with variations in value of the motor speed or n. Within comparatively narrow limits, say ve thousand feet oi altitude, the assumption -that n is a constant is substantially valid. However, when the motor speed changes radically, this assumption is no longer tenable and ifjmade, errors in altitude reading will result. It will be observed that the actual moment applied about .bellows shaft 'it depends upon the pressure diderential between the pres sures in the lSylphon bellows, vtheir effective areas, and the lengths of the arms of the rocker lever 13. The areas of the Sylphon bellows and the length of the arms of the rocker lever i3 are fixed. The eiective force upon the governor control shaft, however, depends upon the position of the pin 55. In order to compensate for the change in the varying ratio ctn/dp, we have found it is necessary to raise the pin 35 as the motor speed becomes less. The correction is a complex one and can .be demonstrated mathematically. We have, however, determined the amount empirically and have given the arm fill and the lever end 50 their correct proportions and vshapes to ,maintain accurate readings over motor speeds between rapproximately 500 revolutions per minute yand 80 .revolutions per minute corresponding to altitudes from zero to fifty thousand feet. It will be understood by those skilled in the art that any suitable motor speed may be chosen. More simply stated, the construction is such that moving the drum @E to a higher altitude so lessens the spring tension that it is necessary to reduce the eiect of the bellows moment as a function of the reduction of mot-or Speed. Continuing description of the operation of our device, we have set a zero valtitude upon the drum 42, the motor having started. We then close the valve 80. It will be understood that as the plane ascends, the pressure in bellows 'il becomes less. The decrease in pressure in the free bellows allows a slight rotation of the rocker lever arm in a clockwise direction. The arm 91 moves the pin 55 to the left as viewed in Fig. 1. This force is exerted against the lever E3, reducing the spring action and decreasing the motor speed, thus registering a higher altitude.

It was assumed that the plane took oi from an air eld having an elevation above sea level of two thousand feet. We are interested in actual altitude from the ground. The barometric de vice measures pressure above sea level. When the plane, for example, reaches an altitude of Vfour thousand feet, it will actually be six thousand feet above sea level. The spring effect is too strong for the bellows effect with the pin 55 positioned for zero altitude on the drum We must, therefore, increase the eiiect of the bellows in accordance with the elevation ci the landing `held above sea level. This is done by moving the arm 100 to the right into one of the holes |02 which are suitably calibrated. Moving the arm to the right will rotate the member 59 in a clockwise direction around its pivot screw 99. This will `raise the pivot screw 50, permitting the springl 6l to raise the right hand side v58 Yof the lever 5l, thus raising the link 56 and hence the pin 55. 'Ihe raising of pin 55 will permit a greater force to be applied against the governor shaft l 6 by reason of torque applied to the bellows shaft ifi. Actually, the movements of the bellows lever arm and the bellows shaft are very minute, their principal function being to transmit moments and torques rather than actual movement. The minute movements involved substantially eliminate hysteresis and fatigue of the metal of the bellows so that accurate readings will result over long periods of time.

Let us continue to follow the operation of the device. It is assumed that `the plane has now reached an altitude of say five thousand feet. This is noted by the pilot. He opens the valve B0, thus equalizing the pressure of the bellows. When this is done, the eiect of the bellows is immediately lostand the motor speed increases, registering an altitude of zero on the galvanorneter, due to the increased voltage generated by the generator 29 occasioned by the increased speed of the motor. The drum 42 is then rotated in a clockwise direction to register rive thousand feet. This rotates the dial yl l 2 in a clockwise direc-tion, bringing the ve thousand calibration to the position formerly occupied by the zero calibration mark. It also rotates shaft 39 in a clockwise direction, rotating shaft llt in a oounterclockwise direction, iessening the spring pressure. This reduces the speed oi the motor and would ordinarily cause the galvanometer needle 55 to move to the left. The rotation of the drum e2 in a clockwise direction, however, also rotates arm 38 in a counter-clockwise direction, thus placing increased resistance in series with the shunt resistance 36. The increased resistance in the shunt permits the galvanometer needle to register maximum amplitude with a reduced voltage, so that we will nd the galvanometer needle indicating an altitude of ve thousand feet above the ground when the drum is moved to five thousand feet and the valve 3G has been opened at the altitude of iive thousand feet. At this point, the valve til is closed. As long as the plane is at an altitude of iive thousand feet the altimeter will register ve thousand feet. Assume now that the plane reaches an altitude of six thousand feet. The pressure in bellows 1| is less than the pressure in bellows 12. This produces a torque about the bellows shaft in a clockwise direction, causing a force on pin 55 tending to rotate the arm 53 in a counter-clockwise direction against the action of spring B2, permitting the motor to slow down, thus producing a lower voltage on the generator 29 and registering a higher altitude by reason of the movement of the galvanorneter needle 35 to the left. Assume now that the plane deviates to an altitude of four thousand feet. in this the bellows pressure in bellows 'il will be greater than that in bellows l2. This will cause a moment in a counter-clockwise direction around bellows shaft 1li. The pin 55 will tend to move to the right as viewed in Fig. 1, adding its force to the spring force, and tending to rotate the gov-1 ernor control shaft in a clockwise direction. This will cause the motor to run at a higher speed, thus generating a higher voltage and moving the galvanometer needle 35 to the right, registering a lower altitude.

It will be recalled that we have applied a correction because of the elevation of the landing field above sea level. Let us assume `that we are to land at another eld whose elevation is at sea level. The pilot will then move the arm itil to the left to a lower altitude by means of the knob It?. Moving the arm lim to the left rotates the member 59 in a counter-clockwise direction, thus lowering the pivot screw {il} and lowering the pin 55. If the plane were flying at an altitude of seven thousand feet, for example, when this oc curred, and the previous setting was two thousand feet for the elevation of the landing eld from which the plane took off, the altimeter would show an altitude of nine thousand feet, whichis the altitude above the landing field at sea level and is the value in which the pilot is interested. The effect is achieved by reducing the eect of the torque applied through the barometricsystem. But for the correction applied, as the plane approached the ground, the increase in pressure in bellows ll would tend to speled up the motor proportionately higher than would be occasioned with the pin in a lower position. This would, therefore, register a lower altitude than the actual altitude. By raising the pin 55, the proportion is so changed that a true measure of altitude above the lower landing iield is achieved. More simply stated, it will be seen that the arrangement is such that we must lower the pin e as higher altitudes are set on the drum 112, in order to compensate for the variation in ratio between changes in motor speed and changes in pressure, which ratio varies as a function of motor speed and hence a function of altitude above the ground. The altitude above the ground, however, is a direct function of the pressure of the surrounding air only when the ground is at sea level. When the ground is above sea level, we must reduce the lowering of the pin 55 by a proportional amount, that is, we must correct the correction in the manner described.

As higher altitudes are reached, the process is repeated, that is, the pressure is equalized and a new altitude is set on the altitude drum and instrument dial and the valve again closed while the plane is flying at the newly set altitude.

It will be seen that we have accomplished the objects of our invention. Our instrument operates by means of small pressure differences, so that the range of motion of the parts is substantially indiscernible. rlhis results in the elimination of hysteresis and fatigue in the metal of the bellows. No correction for pressure in cabin of the airplane is necessary, since any alteration in that -.iessure is automatically compensated for by the use of two bellows. Any range of calibrations any scale is easily achieved. Our instromcnt may be very readily calibrated by means of a listment in the spring tension oi the spring lll, by changes in the motor speed by means of the vari-able resistance 2l in the motor eld, by changes in the resistance in the generator field by means of variable resistance 3l, by changes in the resistance in the galvanometer shunt, or or any of the foregoing means. Accurate altitude readings are obtained over all altitudes from sea level to fifty thousand feet or higher if desired. Qur instrument is capable of measuring altitude diiierences of only fifteen feet at altitudes of twenty-five thousand feet. The altimeters known to the art do not read accurately within one hundred feet at these altitudes. rhree simple adjustments, namely, the setting of the altitude drum, the setting of the landing eld elevation, and the opening and closing of the valve Se are all that is necessary.

it will be observed that our device is of general application and while we have described it with reference to an altimeter, it may be employed as a pressure responsive device in many applica-l tohS.

For example, our invention is adapted to be used with a bombsight such as shown in co-pend ing application of Theodore il. Barth, Ser. No. 335,298, led September 2S, i932, now Patent 2,480,357, in which the altitude drum controls the disk speed or" a bombsight computer corresponding to given values of altitude. When the pilot makes his bombing run at a predetermined altitude, he may encounter disturbances due to updraft or downdraft conditions or last minute changes in bombing altitudes because oi danger ci' collision with other bombing planes coming over the target from diiferent directions or in order to avoid heavy anti-aircraft re which becomes too uncomfortable. ln this case, the valve is closed when the pre-set bombing altitude is reached, and changes in dislr speed, the disk being driven from the motor lll, will automatically be made. in such case, the landing field elevation correction will be calibrated in target elevation as the bombardier is interested in the altitude over the target and not the altitude from sea level, since the bomb must fall from the plane to the target.

it will be further observed that many changes may be made in details without departing from the spirit of our invention. It is, therefore, to be understood that we are not to be limited to the specific details shown and described except within the ambit of the appended claims. Having thus described our invention, what we claim is:

l. in a pressure responsive speed controller, an electric motor, a control circuit for said motor, a switch in the control circuit, a centrifugal speed responsive device actuated by said motor for controlling said switch, a spring for opposing the action of the speed responsive device, pressure responsive means, and means actuated by the pressure responsive means for altering the eiiect oi the spring whereby a variation of pressure upon said pressure responsive means controls the speed of the motor.

2. In a pressure responsive speed controller, a motor, speed control means for said motor, elastic means for actuating said speed control means, speed responsive means for opposing the action of said elastic means, iiuid pressure responsive means, means for applying force from said fluid pressure responsive means to vary the action oi said elastic means, and means for varying the mechanical advantage of said force applying means whereby to alter the effect of force from said pressure responsive means upon said elastic means.

3. An altimeter including in combination, a motor, speed control means for said motor, elastic means for actuating said speed control means, speed responsive means for opposing the action of said elastic means, uid pressure responsive means, means for applying force from the huid pressure responsive means to vary the action of said elastic means and means for varying the mechanical advantage of said force applying means as a function of altitude.

4. An altimeter including in combination, a motor, speed control means for said motor, elastic means for actuating said speed control means, speed responsive means for opposing the action of said elastic means, uid pressure responsive means, means for applying force from the fluid pressure responsive means to vary the action of `said elastic means, means for varying thermechanical advantage of said force applying means as a function of altitude, and a second means for varying the mechanical advantage of said force applying means proportional to the height of the ground vover which the altitude is being measured above sea level.

RICHARD F. WEHRLIN.

ROBERT YOUNG.

REFERENCES CITED The following references are of record in the le of this patent:

M Number Name Date Koppitz May 23, 1918 Paulin Sept. 9, 1919 Smoot' Mar. 30, 1926 Ereme'effi July 10, 1928 Lee June 24, 1930 Bacon Mar. 1, 1932 Jenny July 18, 1939y Newell Feb. 23, 1943 Spencer Oct. 26, 1943 Grooms a Jan. 9, 1945 Ph-lbro'ok et. al. July 20, 19451 FOREIGN PATENTS 1 Country Date i Great Britain Sept. 17, 1931 

